In recent years, there have been ongoing studies on three-dimensional high-density mounting such as package on package (POP) and multi-chip module (MCM) to meet a demand for higher mounting density in electronic parts. When such packages are mounted by BGA with solder balls to increase a density by stacking in the height direction, the solder balls may be unable to withstand the weights of the packages themselves and may collapse. The collapsing of the solder balls may cause short circuit between connection terminals, which is formed by the melting of the solder balls. Thus, it may be hard to achieve the high-density mounting.
In order to solve the above-described problems, JP-A-11-74311 presents to mount with composite balls manufactured from core balls having higher melting point than solder, such as Cu, coated with solder. Having core balls of higher melting point than that of the solder layer prevents the connection terminals from collapsing to the gap height during mounting. Thus, it becomes possible to achieve three-dimensional high-density mounting of the packages.
JP-A-11-74311 presents to manufacture a composite ball by coating the surface of a core ball with solder by plating. The coated layer formed by plating is advantageous in that it is a stable film matching electrically and thermally, suitable for practical use, and providing ease of rolling.
Furthermore, regarding the method of coating the surface of a core ball with solder, JP-A-11-92994 discloses a method of electrolytic plating with horizontally rotatable and hermetically-sealed plating vessel disposed with a cathode in a circumferential part inside the vessel and an anode in the central part inside the vessel, by rotating the vessel at specific high rotational speed. JP-A-11-92994 thereby discloses an improvement to form a solder-plated layer with a uniform thickness without producing cohesion.